## What

When you try to add a user with `.all-contributorsrc` missing, you will get a cryptic error message (see also all-contributors/all-contributors#378):

```
$ all-contributors add darekkay maintenance
Cannot read property 'then' of null
```

This PR displays a meaningful error message in this case.

## Why

So users know what's causing the problem.

## How

Before a contribution is being added, the script checks whether the config file exists. If it doesn't, an error message is being displayed:

```
$ all-contributors add darekkay maintenance
Configuration file not found: C:\projects\test\.all-contributorsrc
```